Various techniques for separating and refining a gas, or for enriching a specific component from a gas have been widely utilized in various fields such as industry, mining and medical treatment with regard to various gases. Among them, a technique for enriching and separating oxygen from air by using a separation membrane has already become practical in a medical apparatus for generating oxygen, and many apparatuses relating to this technique have been known. For example, there are Japanese Patent Kokai No. 58-55310 (apparatus for generating oxygen enriched air), Japanese Patent Kokai No. 59-82903 (apparatus for enriching oxygen), Japanese Patent Kokai No. 59-203705 (device for enriching oxygen), Japanese Patent Kokai No. 62-83022 (gas separation module), Japanese Patent Kokai No. 62-74433 (process for separating gas) and the like. Further, there are many inventions relating to separation membranes used in these apparatuses. For example, there are Japanese Patent Kokai No. 62-30524 (membrane for selective permeation), Japanese Patent Kokai No. 62-74405 (separation membrane) and the like. In general, as a material of a separation membrane, both organic and inorganic compounds have been proposed. In particular, recently, organic polymer compounds have been developed actively and many proposals have been made. Such a membrane is usually composed of a non-porous thin layer (5 to 50 .mu. ) on a porous support (50 to 500 .mu. ) according to various methods. In practice, these non-porous membranes are assembled in a suitable size. Usually, the resulting assembly is referred to as a "module". As a type of the module, plate-and-frame type, spiral type and hollow fiber type are mainly used and each type has respective characteristics. Therefore, they are used properly according to a particular purpose.
When various gases are enriched and separated by using the above module, in order to make such a separation process advantageous over other enrichment and separation processes, it is of importance to improve properties of the above various separation membranes, to improve separation efficiency by using a proper type of the module suitable for a particular use and increasing a packing density of the separation membranes per unit volume of the module, and further, to improve economical efficiency by extending the lifetime of the membrane. On the other hand, when the module is used, it is of importance to reduce running costs for separation. For this purpose, it is of importance to select suitable operating conditions of the module.
The separation operation using a non-porous membrane is carried out by flowing a raw material gas along one surface of the non-porous membrane, while keeping the pressure of the other side of the surface of the membrane lower than that of the raw material gas side. Thereby, a component in the raw material gas is dissolved and diffused in the membrane and is moved toward the lower pressure side. At this time permeability in a membrane varies depending upon a particular component of the gas and therefore the composition of a permeating gas is different from that of the raw material gas. Thus, the composition of the permeating gas is enriched in a certain component, on the other hand, the gas which is not permeated (non-permeating gas) is concentrated.
The running costs in this operation include power costs for heating or cooling the raw material gas and the permeating gas, life time of the module and, if necessary, costs for heating or cooling the raw material gas and the permeating gas, but mainly power costs. In order to reduce such running costs, the following various attempts have been made in the above prior art.
1. A part of the raw material gas is introduced into the permeating gas (Japanese Patent Kokai No. 62-74433).
2. The raw material gas is pressurized and, at the same time, the permeating gas is displaced by suction (Japanese Patent Kokai No. 58-55310).
3. Plural modules are connected in series to form multi-stage modules (Japanese Patent Kokai No. 58-55309).
4. In the above 3, modules are provided so that the area of each module becomes smaller gradually in the downstream direction (Japanese Patent Kokai No. 62-83022).
5. Pressure is kept constant by providing a control valve (Japanese Patent Kokai No. 59-203705).
Further, in order to improve the properties of the above various separation membranes, some proposals have been made. However, as the properties are improved, the costs of the separation membrane itself becomes higher. Therefore, an important factor which effects on the costs of a product as well as the above power costs is how to increase the yield of a product per unit area of the membrane.
In conventional gas separation techniques by using membranes including the techniques as described above, they are limited to feed a raw material gas steadily at a constant pressure from a raw material feeding side, or to evacuate under vacuum from a permeating side, or to carry out both operations simultaneously. In such a process, when the gas flows along the surface of the separation membrane, the more permeable component in the raw material gas is decreased as the surface is closer to the gas outlet and the partial pressure thereof is decreased, which results in insufficient permeation of the gas. Therefore, the desired properties of the separation membrane cannot be sufficiently exerted throughout the entire surface of the membrane, which results in a low product yield per unit area of the separation membrane.